Latin. literally, to 'go among the many'; this was the Roman expression for dying; given the hyper-urbane and secular nature of Roman culture in general (and their rational, nearly atheist outlook, even after adopting Christianity) it should maybe come as little surprise they took a rather dim, unredemptive look at death and subsequently gave it this rather creepy image; originally, from Vergil's Aeneid, when Aeneis walks into the Underworld.
(...embrace the thanatos, love the thanatos...)
Related reading:
Macdonald, R. The burial-places of memory : epic underworlds in Vergil, Dante, and Milton (1987)
O'Hara, J. True names : Vergil and the Alexandrian tradition of etymological wordplay (1996)